Known fuel cell systems include fuel cells that generate power by electrochemically reacting a fuel gas (e.g., hydrogen) supplied to a fuel electrode and an oxidizer gas (e.g., air) supplied to an oxidizer electrode. In order to improve the fuel efficiency, some fuel cells are configured so that excess fuel gas discharged from the fuel electrode is circulated back to the fuel gas supply side through a circulation channel. When air is used as an oxidizer gas, impurities (e.g., nitrogen) in the air permeate from the oxidizer electrode to the fuel electrode. As a result, the impurity concentration in the circulation channel and the fuel electrode tends to increase and the hydrogen partial pressure tends to decrease over time. In this regard, waste gas (i.e., gas containing impurities such as nitrogen, unused hydrogen, and the like) released from the fuel electrode is often discharged out of the fuel cell system through a waste discharge channel connected to the circulation channel by opening and closing a purge valve installed in the waste discharge channel so as to control the amount of impurities.
Such a fuel cell system can be installed on a movable object (e.g., a vehicle). The system supplies electric power generated by the fuel cells to motors for driving the moveable object and for operating auxiliary machineries. Moreover, a secondary battery that compensates for a deficiency of power from the fuel cells can also be incorporated in the fuel cell system.
This type of fuel cell system is usually operated in a power-generating mode in which required power is supplied by the fuel cells. However, if certain switching conditions are satisfied, power generation in the fuel cells is stopped and the system is operated in an idle mode in which the required power is supplied from an energy storing unit to improve the fuel efficiency. In addition to the switching condition, the determination as to whether the system should be switched from the power generation mode to the idle mode includes consideration of whether the performance of the fuel cells that would be stopped if the system is switched to the idle mode will be degraded when the fuel cells are re-started. If the system determines that degradation in performance will occur, the fuel cells are controlled to generate power even when the idle mode would be selected on the basis of the switching conditions. To determine whether degradation in performance will occur, the impurity concentration in the gas supplied to the fuel cells is typically used as the criterion, i.e., the system determines that degradation in performance of fuel cells will occur upon re-start if the impurity concentration is at a predetermined value or higher. For example, Japanese Unexamined Patent Application Publication No. 2005-26054 discloses a fuel cell system of this type.